Field of the Invention
The invention generally relates to handover handling, and more particularly, to apparatuses and methods for handling inter-Radio Access Technology (inter-RAT) mobility.
Description of the Related Art
In a typical mobile communications environment, a user equipment (UE), including a mobile telephone (also known as a cellular or cell phone), a laptop computer with wireless communications capability, or a personal digital assistant (PDA) etc., may communicate voice and/or data signals with one or more service network. The wireless communications between the UE and the service networks may be in compliance with various wireless technologies, such as the Global System for Mobile communications (GSM) technology, General Packet Radio Service (GPRS) technology, Enhanced Data rates for Global Evolution (EDGE) technology, Wideband Code Division Multiple Access (WCDMA) technology, Code Division Multiple Access 2000 (CDMA-2000) technology, Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology, Worldwide Interoperability for Microwave Access (WiMAX) technology, Long Term Evolution (LTE) technology, and others.
Take the LTE technology in compliance with the 3GPP TS 36.331 specification (referred to herein as the TS36.331 specification) as an example. A mobility from E-UTRA (Evolved-Universal Terrestrial Radio Access) procedure is performed for moving a UE in the connected mode with an LTE system to a cell using another RAT, e.g., GSM, GPRS, WCDMA, or CDMA-2000 technology. One of the mobility types may be a handover in which the LTE system may communicate with the target system for allocating radio resources for the UE. Another one of the mobility types may involve a Circuit-Switched (CS) fallback procedure, such as an enhanced CS fallback to CDMA-2000 technology. Alternatively, the CS fallback procedure may be combined with a concurrent handover from the LTE system to a CDMA-2000 system. FIG. 1 is a message sequence chart illustrating a successful mobility from E-UTRA procedure. The E-UTRAN (Evolved-Universal Terrestrial Radio Access Network) of an LTE system prepares and transmits a MobilityFromEUTRACommand message to the UE (step S110), wherein the MobilityFromEUTRACommand message contains configurations of the radio resources allocated for the UE in the target system. When receiving the MobilityFromEUTRACommand message, the UE establishes a connection with the target system according to the configurations of the allocated radio resources in the target system (step S120). After that, the UE releases all radio resources used in the LTE system (step S130), and the mobility from E-UTRA procedure ends. On the other hand, a handover to E-UTRA procedure is performed for transferring a connection between a UE and another Radio Access Network (RAN) to an E-UTRAN. FIG. 2 is a message sequence chart illustrating a successful handover to E-UTRA procedure. The handover to E-UTRA procedure is started by the UE receiving an RRCConnectionReconfiguration message from the E-UTRAN via the RAN which uses an RAT other than the LTE technology and initiates the handover to E-UTRA procedure (step S210). Subsequently, the UE applies the configurations included in the RRCConnectionReconfiguration message (step S220), such as configurations of the radio resources allocated for the UE in the LTE system, security-related configurations, and mobility control information, etc. Next, the UE replies to the E-UTRAN with an RRCConnectionReconfigurationComplete message (step S230), to inform the LTE system of the successful completion of the handover to E-UTRA procedure.